An important process in the fabrication of integrated circuits is the removal of various layers of materials formed on a silicon wafer. Two major etching techniques are in common usage; 1) wet, or chemical etching, wherein a photoresist-patterned silicon wafer is immersed in a chemical solution; and 2) dry, or plasma etching, wherein a wafer is exposed to a plasma containing a gas such as CF.sub.4. The present invention relates mainly to plasma etching.
Plasma etch processes and apparatus are generally well known for etching materials for semiconductor device fabrication. The process begins with application of a masking material, such as photoresist, to a silicon wafer. The masking pattern protects areas of the wafer from the etch process. The wafer is then placed in a plasma reactor ("etcher") and etched. Subsequent steps are determined by the type of device being fabricated. This process is especially valuable for the definition of small geometries.
A common etch process involves the removal of polysilicon ("poly") overlying gate oxide ("oxide"). Among other issues involved, the selectivity of etch between the poly and the underlying gate oxide ("poly:oxide selectivity") should be as high as possible so as to reduce oxide loss.
Using a typical etcher, such as the top-powered, wafer-grounded LAM 490 etcher, poly:oxide selectivities in excess of 100:1 can be achieved with low power (100 Watt) Cl.sub.2 /He plasmas, but can result in severe undercutting or notching of the poly sidewall. On the other hand, helium-rich (greater than 50%), high power (greater than 200 Watts) processes produce vertical sidewalls at the expense of selectivity (less than 30%) and tend to incidentally damage the gate oxide. Lateral etching occurs during the overetch cycle and not during bulk poly removal. Hence, the amount of overetch must be limited.
Irrespective of the particular etch process being employed, it is generally of universal concern to detect when an overlying material ("film") has been completely removed ("cleared"). This is generally accomplished by detecting the presence of the underlying material, now exposed, in the plasma, such as by spectroscopy.
It would also be advantageous to know (i.e., be able to detect) when a desired amount of material, in some cases less than all of the overlying material, has been removed.
U.S. Pat. No. 4,415,402 discloses a method for determining the completion of removal ("clearing") by plasma etching of phosphorous doped silicon dioxide from an underlying substrate. The patent illustrates an endpoint detection technique wherein a specific wavelength, or range of wavelengths, changes in intensity upon clearing the etched material. The patent does not describe a technique for detecting when the film being etched reaches a known, pre-cleared thickness.
U.S. Pat. No. 4,454,001 discloses an interferometric method for measuring etch rate. The method makes use of the phenomenon that device patterns etched into substrates produce diffraction patterns when illuminated. A beam of light is directed onto a region of a substrate being etched. The light reflected from the region forms a diffraction pattern, and the intensity of a diffraction order is detected and recorded as a function of time during the etching process. The etch rate of the substrate is inversely proportional to the period of oscillations in the recorded intensity/time curve.
U.S. Pat. No. 4,680,084 discloses interferometric methods for etch monitoring and thickness measurement. The etch depth of a substrate region undergoing etching is monitored, or the thickness of the region is measured, by impinging the region with light and detecting the intensity of the reflected light.
U.S. Pat. No. 4,687,539 discloses endpoint detection and control of a laser etching process. An excimer laser vaporizes successive layers of a chromium on a region, and a chromium chloride reaction product is formed on the region being etched. A dye laser directed to a zone above the region being etched causes copper chloride in the zone to fluoresce. A narrow band photodetector registers the fluorescence.
U.S. Pat. No. 4,717,446 discloses endpoint detection for etching epitaxially grown silicon. A "monitor" wafer is etched along with "working" wafers. The monitor wafer consists of a silicon substrate having an oxide layer and a polysilicon layer deposited thereon. A laser is used to measure the etch rate of the monitor wafer by measuring reflected light off of the oxide layer.
U.S. Pat. No. 4,936,937 discloses a method of detecting an endpoint of plasma treatment. The use of a spectroscope is disclosed.
U.S. Pat. No. 5,023,188 discloses an interferometric method whereby etched trench depth is determined by shining coherent light onto a wafer surface.
U.S. Pat. Nos. 4,846,928, 4,847,792 and 4,861,419 generally disclose methods of comparing endpoint traces obtained by conventional means with a reference trace in order to detect abnormalities. Reference is made to determining regions of an endpoint curve (i.e., before endpoint, during endpoint, overetch), and taking the slope of the curvature within such regions.
In the references noted above, we generally see that: 1) various techniques and equipment are known for monitoring plasma etching processes; 2) the use of separate illuminating light (e.g., laser) is employed for monitoring; 3) often, test (monitor) wafers are required to characterize and/or monitor etching; and 4) endpoint detection is primarily detected at monitoring clearing of a film (layer) being etched.
Regarding the etching process itself, various etch "recipes" may be used. For example, the flow rate and pressure, electrode spacing, and gaseous species may be varied depending on the material being etched to yield desired selectivities, etch rate and uniformity.